Yup, in a reasonably, but not entirely optimised setup, the spinup time for a new node from scale-up event launching to it being able to serve traffic may take

Yup, in a reasonably, but not entirely optimised setup, the spinup time for a new node from scale-up event launching to it being able to serve traffic may take 2-3 minutes. And trust me, after a couple of mishaps with very aggressive scale-ups, your system will not launch the full demand absorption all at once. In a fully optimised setup each service image is itself 100% preconfigured, and only provisions node secrets during the boot. Even one of these types of nodes takes easily 30-40 seconds from launch event to actually serve traffic: it may join the load balancer just 25 seconds in, but the load balancer will want to see at least two good health checks before allowing any traffic to it. The problem with aggressive upscaling in the depicted scenario is that your plumbing layer is also likely scaled down. Hitting it with a cascade of new nodes has the risk of going all thundering herd, crippling the system for both existing and new nodes.